Polymeric membrane materials for gas separation should not only have good gas separation properties, for example high gas permeability and permselectivity, but should also maintain their intrinsic gas separation properties in complex and harsh environments. Polyimides are attractive membrane materials for gas separation because of their good gas separation and physical properties. Many attempts have been made to modify the chemical structure of polyimides with the aim of obtaining both highly permeable and permselective membrane materials. However most of the polyimides produced either have relative low selectivity or suffer severe ageing and performance decay due to densification and/or plasticization. In order to overcome these problems, cross-linking modifications have been carried out.
Results suggest that cross-linking provides membranes with anti-plasticization properties and good chemical resistance. In addition, the gas permeability/permselectivity relationships of some cross-linked polyimides were higher than the normal trade-off line.
Cross-linking of polyimides can be effected by several methods. For example, one method uses UV light induced photochemical cross-linking reactions in benzophenone-containing polyimides, however the difficulty of implementing the method uniformly on hollow fibre membranes limits its application Another method uses the formation of semi-interpenetrating networks using polymeric blends, and subsequent thermal treatment at elevated temperatures. This method has the potential to yield materials with better anti-plasticization characteristics, however heat treatment is time consuming and is not a preferred process economically. Copolyimides containing strong polar carboxylic acid groups may exhibit reduced plasticization due to hydrogen bonding between the carboxylic acid groups. However, cross-linking reactions between ethylene glycol and carboxylic acid groups in diaminobenzoic acid (DABA) based polyimides commonly occur at around 150° C. A disadvantage of heat treatment at elevated temperatures is the deterioration of the subtle structures of asymmetric membranes and impairment of their gas permeation properties.
Therefore, it is necessary to use low temperature cross-linking methods for the successful modification of asymmetric membranes.
To date, the only simple, practical chemical cross-linking method to modify 6FDA-polyimide asymmetric hollow fibers has been performed by Liu, Wang and Chung (Y.
Liu, R. Wang, T. S. Chung, “Chemical cross-linking modification of polyimide membranes for gas separation”, J. Membr. Sci., 189 (2001) 231). Their process involves immersing fibers into a p-xylenediarine or m-xylenediamine/methanol solution for a short period of time at ambient temperature.